I like to imagine that when Grace first saw Dani in the past, she was struck by some kind of gap moe along the lines of "My personal hero is suddenly my age and now I'm the one protecting her, and wow she's cute
Incest is rightfully wrong if both parties procreate and produce offspring (het only). It's a non-issue in a homosexual relationship or if they adopt children (in het). It's just perceived as strange but that's a cultural thing. Just my 2 cents on the topic of incest.
Incest isn't wrong for anything relating to procreation. It's considered wrong because it's often the result of imbalanced power dynamics and sexual assault...and because people find it icky.